The present invention relates to a cladding tube for nuclear fuel used in a nuclear reactor and, more particularly, to a cladding tube made of a zirconium alloy lined on its inner surface with a zirconium liner. The invention is also concerned with a nuclear fuel element having the above-mentioned cladding tube.
A nuclear fuel element comprises a cladding tube accommodating therein a stack of a plurality of fuel pellets, which are formed by sintering an uranium oxide, thorium oxide, plutonium oxide or a compound thereof, and end plugs sealing both open ends of the cladding tube. Further, in an upper portion of the nuclear fuel element, there are provided a gas storage plenum and a spring for stably holding the fuel pellets.
In the nuclear fuel element having a construction as mentioned above, for the cladding tube such functions are required as to prevent mutual contact and chemical reaction of the fuel pellets with the coolant or moderator and to prevent contamination of the coolant by radioactive fission products emitted from the fuel pellets. Accordingly, the material of the cladding tube is required to have excellent mechanical properties and high corrosion resistance under the operating conditions in the nuclear reactor, as well as a small neutron absorption. From these points of view, zirconium alloys containing zirconium as the major component, such as Zircaloy material, are widely used as the material of the cladding tube. The zirconium alloys have a small neutron absorption cross section, and exhibit only a small reactivity with pure water or steam and suitable strength and ductility at temperatures below 400.degree. C., and so they have excellent properties as a cladding material used under normal condition. However, from the operating experiences up to now it has become clear that at high degree of burn-up the cladding tube causes a stress corrosion cracking because of a synergetic effect of chemical reaction with corrosive fission products (iodine, etc.) and stress caused in the cladding tube by thermal expansion of the fuel pellets. In addition, the outer surface of the cladding tube is locally oxidized by the coolant or moderator of the nuclear reactor.
In order to prevent the aforesaid stress corrosion cracking of cladding tube, a so-called zirconium-lined cladding tube has been developed, in which the cladding tube is lined on its inner surface with a zirconium liner. The zirconium liner is expected to prevent stress corrosion cracking by eliminating contact between the cladding tube and the corrosive fission products and by relieving local stress caused by thermal expansion of the fuel pellets. Thus, the zirconium liner is required to have a high resistance to corrosive fission products and a high ductility to relieve effectively any local stress.
In general, zirconium is less sensitive to the stress corrosion cracking in comparison with zirconium alloys and has high ductility and small neutron absorption cross section, and so it has excellent properties as the liner material.